equal: generated code does not check reference equality

[marcind]

Expected: Generated code returns true if this and that instances are the same, i.e. msg.EqualVT(msg) should be super fast regardless of the fields in the message

Actual: Generated code still walks the whole message hierarchy and compares field-by-field

I believe implementing this should be as simple as replacing the currently generated code

if this == nil {
    return that == nil
} else if that == nil {
    return false
}

with

if this == that {
    return true
} else if this == nil || that == nil {
    return false
}

Could not find a contribution guide, so let me know if you'd like a PR for this or if you would rather make the change yourselves. Thanks for the great project.